STABLE ISOTOPES RESOLVE THE DRIFT PARADOX FOR BAETIS MAYFLIES IN AN ARCTIC RIVER

The colonization cycle hypothesis states that stream ecosystems would become depleted of insects if flying adults did not compensate for dri fting immatures. Using long-term drift and benthic abundance data, we show that a Baetis mayfly nymph population moves downstream during dev elopment in the Kuparuk River in arctic Alaska. Baetis relative benthi c abundance decreased from early to late season in an upstream unferti lized river section, while simultaneously increasing in the downstream fertilized section. Baetis nymphs drifted significantly more in the u pstream unfertilized section, compared to the downstream fertilized se ction where food was more abundant. Approximately one-third to one-hal f of the nymph population drifted at least 2.1 km downstream during th e arctic summer. A stable isotope tracer experiment and mathematical m odels show that about one-third to one-half of the adult Baetis popula tion flew 1.6-1.9 km upstream from where they emerged. These results p rovide a quantitative test of the colonization cycle for the dominant grazer/collector in the Kuparuk River. Quantifying the colonization cy cle is essential to understanding stream ecosystem function because of fspring of downstream insects are needed for nutrient cycling and carb on processing upstream. Since downstream drift and upstream flight are important components in recovery of streams from disturbances, our re sults provide a quantitative method for predicting recolonization rate s from downstream, essential to estimating recovery.